To classify an electrical machine, use may be made inter alia of what is referred to as the power-to-weight ratio that expresses the power able to be supplied by the machine in relation to a weight and is generally given in kW/kg. Although power-to-weight ratios of orders of magnitude of up to 1 kW/kg are sufficient for many technical applications, electrical machines including power-to-weight rations of at least 20 kW/kg are required for electrification in aviation, for example.
To increase the power-to-weight ratio of electrical machines, the current coverage in the stator of the machine or the current density in the stator windings may be increased, for example. The increase results in less conductor material that is required in the turns at the same power, as a result of which the weight of the machine is reduced with the power staying the same.
To increase the current density from normal values, such as for example 2-10 A/mm2, to orders of magnitude of 25-100 A/mm2, a first approach may involve cooling the copper windings that form the stator turns during operation. The copper windings may be brought into direct contact with a coolant, or the coppers windings may be configured to be internally hollow and the coolant is pumped directly through the cavity that is formed. In both cases, a comparatively large contact surface is obtained between conductor and coolant, and therefore an increased cooling effect, such that a comparatively large amount of heat is able to be evacuated. This is reflected in the fact that the electrical lines are able to transport higher currents. A second approach for increasing the current density involves using cryogenically cooled, possibly even superconductive electrical conductors with electrical resistance that decreases significantly with the ambient temperature. The losses in the conductor are consequently reduced, resulting in comparatively high current densities.
In both approaches, that are based on cooling the respective electrical conductor, there is the disadvantage that an increased effort is required in order to cool the conductor. Furthermore, in the case of the internally cooled hollow conductor, electrical losses are comparatively high, and the efficiency is consequently low.